Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskPt( hkl ) Studies

The first results of a study of the oxygen reduction reaction on a rotating ring−disk electrode using single-crystal Pt disk electrodes in alkaline solution are presented. The order of activity of Pt( hkl ) in 0.1M KOH increased in the sequence (100) < (110) < (111) for both oxygen and peroxide reduction. These differences are attributed to the structure sensitivity of hydroxyl anion (OH - ) adsorption on Pt( hkl ) and its inhibiting (site blocking) effect on oxygen kinetics. Two different types of OH ads occur on Pt(100) and Pt(110), a reversible OH rv, and an irreversible OH ir, while only the reversible form is formed on Pt(111). Very small fluxes ( I R / N ≪ 0.01 I D ) of peroxide were detected at the ring at potentials where the surfaces have only the reversible form of OH ads . More peroxide ( I R / N < 0.03 I D ) was found with the Pt(100) and Pt(110)surfaces in the potential region where irreversible adsorption occurs. The mechanism of O 2 reduction was also affected by adsorbed hydrogen with increased formation of peroxide ions in this potential region; correspondingly, adsorbed hydrogen was found to have an inhibiting effect on peroxide reduction, the effect decreasing in the order (111) > (100) ≫ (110).